1. Field
The present disclosure relates to a process for the bulk synthesis or synthesis in “semi-bulk” concentrated medium of diene elastomers with a high degree of conversion. The disclosure applies especially to the continuous production of diene elastomers.
2. Description of Related Art
During a bulk polymerisation, the reaction medium lacks solvent or diluent. The monomers are then polymerised in the absence of any solvent. The components of the reaction medium in contact are the monomers, the polymer and the constituents of the catalytic system enabling the polymerisation. Polymerisation in “semi-bulk” concentrated medium itself uses only a small amount of solvent when compared with solution polymerisation. It is a polymerisation which proceeds in a solution that is highly concentrated in polymer and weakly concentrated in solvent.
The absence of solvent or of diluent, or alternatively the presence of a reduced amount of solvent or of diluent in the reaction medium has appreciable economic and environmental advantages. The use of solvent or diluent in polymerisation involves the latter being separated from the polymer prepared after the polymerisation step. The extracted solvent or diluent is generally recycled in order to be reused. The cost of the extraction and recycling of the solvent or diluent significantly increases the synthesis cost of the polymer.
Thus, bulk or semi-bulk polymerisation offers many advantages. This type of polymerisation is especially of economic interest, above all as regards the investment costs. Specifically, the size of the items of equipment, in particular those outside the reaction part, is reduced due to the small amount of solvent used.
This reduction of the amount of solvent or diluent also gives access to higher production efficiency of the reactors.
In addition, this reduction or omission of the solvent or diluent entails a decrease in the amount of energy (in the form of vapour for removal of the solvent by steam stripping) to separate the elastomer from the solvent, and also a decrease in the flow of solvent to be treated. All these characteristics lead to a decrease in the energy costs of the polymerisation process.
Moreover, the amount of catalyst is generally reduced, which lowers the cost of the starting materials.
Besides these economic aspects, the reduction of the flow of solvent has an environmental impact since it leads to a reduction of the emission losses and a decrease in energy consumption.
Several processes for the bulk or semi-bulk polymerisation of conjugated dienes have been proposed in the past.
Mention may be made, for example, of patent EP 0 127 236 B1, which describes the bulk polymerisation of butadiene. This is a continuous catalytic polymerisation of butadiene, performed without or with a small proportion of solvent, ranging up to 2% by mass relative to the mass of the monomer. A neodymium-based catalytic system and butadiene feed a polymerisation reactor/extruder. The polymerisation is performed with a monomer content at the reactor/extruder inlet of between 66% and 96% of the total mass of solvent+monomer. In this type of reactor, the variations in residence time are limited. This major drawback does not give access to reactions that require long residence times. Nor does it afford flexibility in the execution of the reaction. In addition, with this type of reactor/extruder, it is difficult to have a constant polymerisation temperature, which may, depending on the catalytic system used, have an impact on the properties of the polymers synthesised, especially their microstructure or their macrostructure.
Another document, patent application WO 2005/087 824, describes a process for the polymerisation of conjugated dienes, which may be performed in continuous or batch mode with less than 50% by mass of solvent. It uses a catalytic system based on lanthanide or cobalt. The polymerisation is performed in a first reactor up to a degree of conversion of not more than 20%. The polymer is discharged and transferred into a second reactor in which devolatilization may be performed.
Limitation to a low degree of conversion of 20% leads to low production efficiency of this process. In addition, the unconverted monomer must be extracted and treated in order to be reintroduced into the first reactor. This treatment takes place on large volumes of unconverted monomer. This aspect of the process makes it sparingly attractive due to its economic and energy impact on the cost of the process.
Limitation to a low degree of conversion of the bulk or semi-bulk polymerisation is also dictated by the appearance of a physical phenomenon that might degrade the nature of the polymer obtained and give rise to uncontrollable situations in an industrial production. Specifically, the vaporization of part of the liquid monomer, due to the effect of the energy of the polymerisation reaction, leads, at and above a certain polymer concentration in the reaction medium, to excessive expansion of the reaction medium. Under such operating conditions, it is no longer possible to control the heat of the reaction and the polymer may thereby be denatured.
Solutions have already been provided in the past to overcome this problem of expansion of the polymerisation medium, especially that of reducing the degree of conversion and of maintaining it below the limit above which the phenomenon is observed at a given temperature.
Thus, U.S. Pat. No. 3,770,710, which is directly aimed at overcoming the expansion phenomenon, proposes a process for the continuous polymerisation of conjugated dienes in two steps. The first step consists of a polymerisation of a liquid butadiene at low temperature, below 50° C., to reach a degree of conversion of between 20% and 40%. The temperature is maintained by controlled evaporation of the liquid phase. The second step consists in continuing the polymerisation and drying of the polymer obtained in a zone separate from the first, especially an extruder, at a higher temperature ranging from 50° C. to 150° C.
Although the expansion phenomenon is avoided, the process described in the said document has drawbacks similar to those of other existing processes, especially a low production efficiency of the process due to a degree of conversion of less than 40%, and also low attractiveness due to the large volumes of unconverted monomer to be manipulated, which leads to a large economic and energy impact.